This invention relates to a method of synchronizing network elements in a synchronous digital communications network, to a network element for a synchronous digital communications network, and to a network management facility.
The network elements of a synchronous digital communications network operate according to the recommendations of the ITU-T for synchronous digital hierarchy (SDH) or for the Synchronous Optical Network (SONET). By xe2x80x9cnetwork elementsxe2x80x9d, add-drop multplexers, crossconnects, or line multiplexers are understood. During the transmission of information in such a communications network it is essential that all network elements operate in synchronism. This is achieved by mutual synchronization of the network elements or by master-slave synchronization, in which each network element derives a clock signal from a received message signal and synchronizes its internal clock generator with this external clock signal. One or more primary reference clock generators supply a highly accurate reference clock for the network.
To improve the selection of a message signal as a timing reference, a synchronization status message (SSM) was introduced, which is transmitted as part of the message signal. The SSM indicates the quality of the reference clock to which the transmitting network element is synchronized. The selection of a timing reference is based on the SSM: A network element selects that information signal as a timing reference whose SSM indicates the highest clock quality. As SSM, the message xe2x80x9cDNUxe2x80x9d (xe2x80x9cdo not use for synchronizationxe2x80x9d) was defined, which is to be transmitted in the return direction to the network element whose information signal was selected as the timing reference.
During synchronization, care must be taken to ensure that no timing loops are created, i.e., that two network elements do not select each other as a synchronization clock source. The risk that timing loops are created is particularly great if two or more parallel transmission paths are present between two network elements. Such parallel transmission paths are also referred to as xe2x80x9cbundlesxe2x80x9d. By the use of the SSM xe2x80x9cDNUxe2x80x9d alone, the creation of timing loops cannot be avoided in all possible situations.
To avoid timing loops, European Patent Application EP 0 723 344 proposes to define two classes of interface devices. Only information signals that are received at an interface device of the first class may be selected as a timing reference, while information signals that are received at interface devices of the second class must be ignored in selecting a reference clock. By suitable configuration of the network, e.g., by connecting only interfaces of the first class to interfaces of the second class, timing loops can be avoided. However, this solution is prone to error, since it depends on the correct network configuration. Particularly if bundles of parallel transmission paths are present, errors may creep into the configuration, which then result in the creation of timing loops.
European Patent Application EP 0 849 904 proposes another way of avoiding timing loops. The selection of a reference clock is made in a central clock generator of a network node. The selected clock source is communicated to a central network management facility which then instructs each network element of the node that receives its reference clock from the central clock generator which SSM to send at which of its outputs. In this way, the SSM xe2x80x9cDNUxe2x80x9d can be sent at all outputs involving the risk of the creation of timing loops, whereby the creation of timing loops is effectively prevented. However, this method is complex, because the network management facility must explicitly assign an SSM to each output of each network element. This method, too, depends essentially on the correct configuration of the network. In addition, several clock interfaces with means for transmitting an SSM must be provided between the clock generator and the network elements. This, too, adds to the complexity and cost of equipment, and is not provided for in currently available network elements.
It is an object of the invention to provide a method of synchronizing a network element which reduces the risk of the creation of timing loops, particularly in the presence of parallel transmission paths, but which manages with simple technical means. Further objects of the invention are to provide a network element and a network management facility which are suitable for carrying out the method.
The first-mentioned object is attained by the features of claim 1, and the further objects are attained by the features of claims 6 and 7, respectively. Further advantageous aspects of the invention are defined in the dependent claims.